This goes to heart of most high handicappers problem, in a word "Balance", you have to have balance, lose it and your body begins to make adjustment, you can't stop it. Bend over and lean into your toes, once you start to fall, you will automatically straighten up, it is reflex, you have spent a lifetime learning not to fall down. So straightening up in a golf swing usually means your balance is off towards the toes, can be several things, bad address, started out bent over too far, or you thrust your right side out towards the ball on the downswing, again weight goes to the toes, maybe you threw the right shoulder out and over, this also puts the weight out on the toes, the idea here is without seeing your swing it is a tough call, you will have to find out which part of the swing is causing it, but losing spine angle is ALWAYS balance related, if the weight was on the heels you have the opposite problem, you bend over more.